The present invention relates generally to adjusting the air/fuel ratio in the cylinders of an internal combustion engine to control automotive emissions. More particularly, the present invention relates to a method and system for estimating an amount of oxidants stored in catalyst for use in controlling the engine air/fuel ratio.
To minimize the amount of emissions exhausted into the atmosphere, modern automotive vehicles generally include one or more catalytic converters, or emission control devices, in the exhaust system of the vehicle. These emission control devices store oxygen and NOx (collectively, oxidants) from the vehicle exhaust stream when the engine is operated with a relatively lean air/fuel ratio. On the other hand, when the engine is operated with a relatively rich air/fuel ratio, they release the stored oxygen and NOx, which then react with the HC and CO produced by the engine. In this way, the emission of both NOx and hydrocarbons (HC and CO) into the atmosphere is minimized.
The inventors have recognized a disadvantage with conventional air-fuel ratio control systems. In particular, the inventors have recognized that these systems attempt to maintain the engine at stoichiometry (or another desired air-fuel ratio). However, this has the disadvantage that engine air-fuel control is decoupled from the state of oxidant storage of the emission control device. The convention system relies on air-fuel feedback to compensate for this oversight.
To overcome disadvantages with prior approaches, the inventors have developed a method for controlling the engine air-fuel ratio to maintain the oxidant level stored in the emission system at a desired set-point level. However, the inventors have further recognized that to implement such a system, an accurate method of determining the amount of oxidants stored in the emission control device should be used. In particular, known estimators for determining oxidants retained in an emission control device (for example, see EP 598,917) ignore the reductant that is produced at the same time the oxidants are produced. Further, such known estimates degrade with time since engine operating and catalyst performance vary.
The above disadvantages are overcome by method of estimating a change in an amount of oxidants stored in a catalyst of an exhaust system coupled to an internal combustion engine. The method comprises:
determining a first amount of oxidants available for being stored in the catalyst and a second amount of oxidants required to oxidize reductants being produced by the engine based on at least an engine air/fuel ratio; and
estimating a third amount of oxidants that are retained by or released from the catalyst over a period based on said first amount of oxidants available for being stored in the catalyst and said second amount of oxidants required to oxidize hydrocarbons being produced by the engine.
According to the present invention, it is possible to accurately determine oxidant storage since the effect of both the oxidants and reductants produced by the engine are included in the calculations. In this way, an accurate estimate of catalyst oxidant storage can be used to accurately adjust operating to keep the catalyst with a desired amount of oxidant storage. inadvertent engine excursions that produce excess oxidants, or excess hydrocarbons, can be minimized by the catalyst.